dt-bindings: Document STM32 ADC DT bindings

This patch adds documentation of device tree bindings for the STM32 ADC.
It's based on linux-v4.18-rc* dt-bindings, at the time of writing:
- Documentation/devicetree/bindings/iio/adc/st,stm32-adc.txt

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

diff --git a/doc/device-tree-bindings/adc/st,stm32-adc.txt b/doc/device-tree-bindings/adc/st,stm32-adc.txt
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+STMicroelectronics STM32 ADC device
+
+STM32 ADC is a successive approximation analog-to-digital converter.
+It has several multiplexed input channels. Conversions can be performed
+in single, continuous, scan or discontinuous mode. Result of the ADC is
+stored in a left-aligned or right-aligned 32-bit data register.
+Conversions can be launched in software or using hardware triggers.
+
+The analog watchdog feature allows the application to detect if the input
+voltage goes beyond the user-defined, higher or lower thresholds.
+
+Each STM32 ADC block can have up to 3 ADC instances.
+
+Each instance supports two contexts to manage conversions, each one has its
+own configurable sequence and trigger:
+- regular conversion can be done in sequence, running in background
+- injected conversions have higher priority, and so have the ability to
+  interrupt regular conversion sequence (either triggered in SW or HW).
+  Regular sequence is resumed, in case it has been interrupted.
+
+Contents of a stm32 adc root node:
+-----------------------------------
+Required properties:
+- compatible: Should be one of:
+  "st,stm32f4-adc-core"
+  "st,stm32h7-adc-core"
+  "st,stm32mp1-adc-core"
+- reg: Offset and length of the ADC block register set.
+- interrupts: One or more interrupts for ADC block. Some parts like stm32f4
+  and stm32h7 share a common ADC interrupt line. stm32mp1 has two separate
+  interrupt lines, one for each ADC within ADC block.
+- clocks: Core can use up to two clocks, depending on part used:
+  - "adc" clock: for the analog circuitry, common to all ADCs.
+    It's required on stm32f4.
+    It's optional on stm32h7.
+  - "bus" clock: for registers access, common to all ADCs.
+    It's not present on stm32f4.
+    It's required on stm32h7.
+- clock-names: Must be "adc" and/or "bus" depending on part used.
+- interrupt-controller: Identifies the controller node as interrupt-parent
+- vref-supply: Phandle to the vref input analog reference voltage.
+- #interrupt-cells = <1>;
+- #address-cells = <1>;
+- #size-cells = <0>;
+
+Optional properties:
+- A pinctrl state named "default" for each ADC channel may be defined to set
+  inX ADC pins in mode of operation for analog input on external pin.
+
+Contents of a stm32 adc child node:
+-----------------------------------
+An ADC block node should contain at least one subnode, representing an
+ADC instance available on the machine.
+
+Required properties:
+- compatible: Should be one of:
+  "st,stm32f4-adc"
+  "st,stm32h7-adc"
+  "st,stm32mp1-adc"
+- reg: Offset of ADC instance in ADC block (e.g. may be 0x0, 0x100, 0x200).
+- clocks: Input clock private to this ADC instance. It's required only on
+  stm32f4, that has per instance clock input for registers access.
+- interrupt-parent: Phandle to the parent interrupt controller.
+- interrupts: IRQ Line for the ADC (e.g. may be 0 for adc@0, 1 for adc@100 or
+  2 for adc@200).
+- st,adc-channels: List of single-ended channels muxed for this ADC.
+  It can have up to 16 channels on stm32f4 or 20 channels on stm32h7, numbered
+  from 0 to 15 or 19 (resp. for in0..in15 or in0..in19).
+- st,adc-diff-channels: List of differential channels muxed for this ADC.
+  Depending on part used, some channels can be configured as differential
+  instead of single-ended (e.g. stm32h7). List here positive and negative
+  inputs pairs as <vinp vinn>, <vinp vinn>,... vinp and vinn are numbered
+  from 0 to 19 on stm32h7)
+  Note: At least one of "st,adc-channels" or "st,adc-diff-channels" is required.
+  Both properties can be used together. Some channels can be used as
+  single-ended and some other ones as differential (mixed). But channels
+  can't be configured both as single-ended and differential (invalid).
+- #io-channel-cells = <1>: See the IIO bindings section "IIO consumers" in
+  Documentation/devicetree/bindings/iio/iio-bindings.txt
+
+Optional properties:
+- dmas: Phandle to dma channel for this ADC instance.
+  See ../../dma/dma.txt for details.
+- dma-names: Must be "rx" when dmas property is being used.
+- assigned-resolution-bits: Resolution (bits) to use for conversions. Must
+  match device available resolutions:
+  * can be 6, 8, 10 or 12 on stm32f4
+  * can be 8, 10, 12, 14 or 16 on stm32h7
+  Default is maximum resolution if unset.
+- st,min-sample-time-nsecs: Minimum sampling time in nanoseconds.
+  Depending on hardware (board) e.g. high/low analog input source impedance,
+  fine tune of ADC sampling time may be recommended.
+  This can be either one value or an array that matches 'st,adc-channels' list,
+  to set sample time resp. for all channels, or independently for each channel.
+
+Example:
+       adc: adc@40012000 {
+               compatible = "st,stm32f4-adc-core";
+               reg = <0x40012000 0x400>;
+               interrupts = <18>;
+               clocks = <&rcc 0 168>;
+               clock-names = "adc";
+               vref-supply = <&reg_vref>;
+               interrupt-controller;
+               pinctrl-names = "default";
+               pinctrl-0 = <&adc3_in8_pin>;
+
+               #interrupt-cells = <1>;
+               #address-cells = <1>;
+               #size-cells = <0>;
+
+               adc@0 {
+                       compatible = "st,stm32f4-adc";
+                       #io-channel-cells = <1>;
+                       reg = <0x0>;
+                       clocks = <&rcc 0 168>;
+                       interrupt-parent = <&adc>;
+                       interrupts = <0>;
+                       st,adc-channels = <8>;
+                       dmas = <&dma2 0 0 0x400 0x0>;
+                       dma-names = "rx";
+                       assigned-resolution-bits = <8>;
+               };
+               ...
+               other adc child nodes follow...
+       };
+
+Example to setup:
+- channel 1 as single-ended
+- channels 2 & 3 as differential (with resp. 6 & 7 negative inputs)
+
+       adc: adc@40022000 {
+               compatible = "st,stm32h7-adc-core";
+               ...
+               adc1: adc@0 {
+                       compatible = "st,stm32h7-adc";
+                       ...
+                       st,adc-channels = <1>;
+                       st,adc-diff-channels = <2 6>, <3 7>;
+               };
+       };